70ª Defesa de Dissertação de Mestrado, de Carlos Ivan Henao Osorio

Data Defesa: 10/11/2014 - Horário: 14h

Local: Sala 303 - 3º andar - Bloco B

Security of information storage and communication has become a crucial issue for the development and economic competitiveness of any nation. Cryptographic methods that make use of quantum technology are by far more secure than their classical counterparts, thanks to the use of quantum principles that allow detecting the presence of an eavesdropper. We can say that quantum cryptography, based on quantum systems for key distribution (QKD - quantum key distribution) is ready for commercial technology. In April of this year, the Chinese government started to install a 2000 km link of optical fibers between Beijing and Shanghai aimed to perform quantum communication, including QKD [J. Qiu, Nature 508, 441 (2014)]. In addition, a recent agreement between Los Alamos National Laboratory and Whitewood Encryption Systems, Inc. promises to put QKD technology “at the hand of the average person” [http://phys.org/news/2014-09-quantum-key-technology-everyman.html]. In this work we present new results concerning Two-Way Quantum Key Distribution (TWQKD) protocols. The peculiar feature of TWQKD is that the bits constituting the cryptographic key are transmitted two times (forward and backward) through the quantum channel, instead of only one as in traditional One-Way Protocols, e.g., BB84. Paradigmatic protocols of this kind are LM05 and the "Ping-Pong" protocol. Although initially focused on performing secure direct communication, if used for QKD they can have some pros when compared to One-Way QKD. In particular, TWQKD protocols may be deterministic, meaning that it is possible to decode all the encoded bits and hence to make a more efficient use of the resources. Nonetheless, One-Way protocols can offer a larger security level in the sense that secret keys can be extracted in more noisy environments. We explore how to construct a TWQKD protocol combining efficiency and robustness to noise. Thereby we deduce a protocol and we prove its security for collective attacks. Remarkably, it allows us to show that no deterministic TWQKD protocol can be more robust to noise than the deduced one. Furthermore, we compare the amount of classical communication that it requires with respect to other protocols. On the other hand, we also analyse the performance of TWQKD protocols beyond the typical model of the quantum-depolarizing channel. Regarding this issue we find that the extracted information by an eavesdropper could be reduced if a channel that produces asymmetric noise is employed. We hope that the theoretical results presented here can be experimentally implemented (in near future) in order to verify their usefulness in practical scenarios of security communication.